Solid polymer electrolyte composite membrane comprising plasma etched porous support

ABSTRACT

A solid polymer electrolyte composite membrane and method of manufacturing the same. According to one embodiment, the composite membrane comprises a rigid, non-electrically-conducting support, the support preferably being a sheet of polyimide having a thickness of about 7.5 to 15 microns. The support has a plurality of cylindrical pores extending perpendicularly between opposing top and bottom surfaces of the support. The pores, which preferably have a diameter of about 0.1 to 5 microns, are made by plasma etching and preferably are arranged in a defined pattern, for example, with fewer pores located in areas of high membrane stress and more pores located in areas of low membrane stress. The pores are filled with a first solid polymer electrolyte, such as a perfluorosulfonic acid (PFSA) polymer. A second solid polymer electrolyte, which may be the same as or different than the first solid polymer electrolyte, may be deposited over the top and/or bottom of the first solid polymer electrolyte.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit under 35 U.S.C. 119(e) of U.S. Provisional Patent Application Ser. No. 60/613,769, filed Sep. 28, 2004, the disclosure of which is incorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The U.S. Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of Contract No. DE-FG36-04GO13029 awarded by the Department of Energy.

BACKGROUND OF THE INVENTION

The present invention relates generally to solid polymer electrolyte membranes of the type suitable for use in electrochemical devices and relates more particularly to a novel such membrane.

Electrochemical devices of the type comprising a solid polymer electrolyte membrane (PEM) sandwiched between a pair of electrodes are well-known, such electrochemical devices finding applications as, for example, fuel cells, electrolyzers, sensors, gas concentrators, gas compressors, supercapacitors, ultracapacitors and industrial electrochemical process units.

A common type of solid polymer electrolyte membrane consists of a homogeneous perfluorosulfonic acid (PFSA) polymer, said PFSA polymer being formed by the copolymerization of tetrafluoroethylene and perfluorovinylether sulfonic acid. See e.g., U.S. Pat. No. 3,282,875, inventors Connolly et al., issued Nov. 1, 1966; U.S. Pat. No. 4,470,889, inventors Ezzell et al., issued Sep. 11, 1984; U.S. Pat. No. 4,478,695, inventors Ezzell et al., issued Oct. 23, 1984; U.S. Pat. No. 6,492,431, inventor Cisar, issued Dec. 10, 2002, all of which are incorporated herein by reference. A commercial embodiment of a perfluorosulfonic acid polymer PEM is available from DuPont (Wilmington, Del.) as NAFION® PFSA polymer.

Although PFSA PEMs function in a generally satisfactory manner in electrochemical devices, there nonetheless remains room for improvement in certain properties of PFSA PEMs. For example, one common difficulty associated with PFSA PEMs is a lack of mechanical strength, resulting in a tendency for the PFSA PEMs to tear, especially when being handled (such as during assembly of an electrochemical cell) or in stressed areas where compression is applied thereto (such as in peripheral areas of PEMs sealed under pressure to other electrochemical cell components). Such a lack of mechanical strength also often leads to electrical shorting, which results in premature failures during cell operation as the typical porous electrodes in contact with the PEM have a tendency to penetrate the softened PEM. This problem of shorting is even greater when the membrane is made thin (e.g., less than 25 microns) in order to decrease membrane resistance.

Because the tendency to tear and to short is greatest when the PFSA PEMs are wet (especially at elevated temperatures) and because the PFSA PEMs must be wet in order to function properly, one approach to this problem has been to assemble electrochemical cells with dry PEMs and then to subject the PEMs to a humidification process. This approach, however, has its own shortcomings. One such shortcoming is that the dry assembly requires special moisture-free facilities, such as a “dry room.” Another such shortcoming is that the humidification process is time-consuming. Still another such shortcoming is that the humidification process typically results in the PEM swelling in a non-uniform manner, thereby creating stress in some areas of the PEM, as well as in other components of the cell that are in contact with the PEM, and introducing irregularities in the contact pressure applied over the entire active surface area of the PEM. (When the contact pressure is not uniform over the entire active surface area of the PEM, the performance of the electrochemical cell is adversely affected.) As can readily be appreciated, such irregularities are amplified where humidification is applied to a plurality of PEM-containing fuel cells arranged in a stack.

Moreover, if the PEM is subjected to variable conditions of humidity (e.g., alternating wet and dry intervals during periods of use and non-use, respectively), the membrane will undergo additional dimensional changes as it swells when wet and shrinks when dry. Such dimensional changes cause further stress to the PEM and to the other cell components, all of which are tightly packed together. If sufficiently great, such stress results in damage to the PEM and/or to the cell components in contact therewith. Pinholes/microcracks have a tendency to form along the edges where one side of the membrane is heavily compressed by the fixture while the other side can still partially swell.

One approach that has been taken to address the aforementioned problem of low mechanical strength of PFSA PEMs has been to cross-link the membrane polymer. Such cross-linking reduces the swelling of the membrane when wet which, in turn, reduces the deterioration of the mechanical strength of the membrane when wet. Unfortunately, however, such cross-linking tends to make the membrane undesirably brittle under dry conditions.

Another approach to this problem is disclosed in U.S. Pat. No. 6,635,384, inventors Bahar et al., which issued Oct. 21, 2003, and which is incorporated herein by reference. In the aforementioned '384 patent, there is described a composite membrane that comprises a microporous sheet, said microporous sheet preferably being an expanded polytetrafluoroethylene (ePTFE) membrane, said ePTFE membrane preferably being formed by stretching a sheet of polytetrafluoroethylene (PTFE) until pores are formed therein. The structure defining the pores of the microporous sheet is then at least partially covered with a functional material selected from (i) inorganic particulate; (ii) metal; and (iii) an organic polymer. In addition, the pores of the sheet are then at least partially filled with polymer electrolyte selected from (i) polymer compositions that contain metal salts; (ii) polymeric gels that contain electrolyte; and (iii) ion exchange resins, such as PFSA.

One disadvantage that has been noted by the present inventors regarding the foregoing composite membrane is that the pores of the expanded polytetrafluoroethylene (ePTFE) sheet tend to follow a tortuous pathway between opposing surfaces of the ePTFE sheet, as opposed to following a direct or straight pathway between opposing surfaces. As a result of these tortuous pathways, protons conducted through the pores (by means of the polymer electrolyte that is disposed within the pores) have to travel considerably longer pathways through the membrane than merely the thickness of the membrane. Such longer pathways result in a reduction in the conductivity of the membrane and an increase in the resistivity of the membrane.

Another disadvantage that has been noted by the present inventors regarding the foregoing composite membrane is that a microporous sheet of ePTFE possesses only slightly better mechanical strength than a PFSA PEM. Consequently, the foregoing composite membrane is not significantly stronger than a PFSA PEM and is subject to the same types of shortcomings discussed above in connection with PFSA PEMs.

Still another disadvantage that has been noted by the present inventors regarding the foregoing composite membrane is that the stretching process that is used to expand the PTFE sheet to create the desired pores tends to result in a fairly uniform yet random distribution of pores throughout the sheet and cannot be tailored to control the positioning or concentration of pores in particular regions of the sheet. This is unfortunate because certain regions of the membrane, such as the membrane active area edges/corners or the membrane contact area with the current collector, are typically subjected to greater stresses than other regions. Consequently, pores in these regions of high stress undesirably diminish membrane strength in those regions where membrane strength is needed most.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a novel solid polymer electrolyte membrane of the type that is suitable for use in electrochemical devices, such as, but not limited to, fuel cells, electrolyzers, sensors, gas concentrators, gas compressors, supercapacitors, ultracapacitors and industrial electrochemical process units.

It is another object of the present invention to provide a solid polymer electrolyte membrane of the type described above that overcomes at least some of the drawbacks discussed above in connection with existing solid polymer electrolyte membranes.

Therefore, according to one aspect of the invention, there is provided a solid polymer electrolyte composite membrane, said solid polymer electrolyte composite membrane being prepared by a method comprising the steps of (a) providing a non-electrically-conductive support, said non-electrically-conductive support having opposing top and bottom surfaces; (b) plasma etching a plurality of pores in said non-electrically-conductive support, said pores extending directly from said top surface to said bottom surface; and (c) at least partially filling at least some of said pores with a first solid polymer electrolyte.

In a preferred embodiment, the non-electrically-conductive support is a polyimide membrane having a thickness of about 5 μm to 50 μm. The pores have a diameter of about 0.1 μm to 200 μm, preferably 0.5 μm to 10 μm, and are arranged in a defined pattern, such as in a uniform hexangular pattern or in a pattern in which fewer pores are located in areas of higher membrane stress and more pores are located in areas of lower membrane stress. A solid polymer electrolyte, such as PFSA polymer, fills the pores. Additional solid polymer electrolyte, which may be the same as or different than that filling the pores, may be applied to one or both of the top and bottom surfaces of the solid polymer electrolyte filling the pores.

The present invention is also directed to a method of preparing a solid polymer electrolyte composite membrane. According to one aspect of the invention, such a method comprises the steps of (a) providing a non-electrically-conductive support, said non-electrically-conductive support having opposing top and bottom surfaces; (b) plasma etching a plurality of pores in said non-electrically-conductive support, said pores extending directly from said top surface to said bottom surface; and (c) at least partially filing at least some of said pores with a first solid polymer electrolyte.

In a preferred embodiment, the plasma etching step comprises sputtering a first thin layer of aluminum onto a silicon wafer; spin coating a layer of what will become the support, preferably polyimide, onto the first thin layer of aluminum; sputtering a second thin layer of aluminum onto the support precursor; spin coating a layer of photoresist material onto the second thin layer of aluminum; imaging and developing the photoresist layer, thereby creating pores in the photoresist layer that expose the underlying second aluminum layer; etching the exposed areas of the second thin layer of aluminum, thereby creating pores in the second thin layer of aluminum to expose the underlying support layer; removing the photoresist layer; plasma etching the exposed areas of the support layer; and removing the aluminum layers from the etched support.

The present invention is also directed to membrane electrode assemblies incorporating the above-described composite membranes and to electrochemical devices incorporating the above-described composite membranes, such electrochemical devices including, but not being limited to, fuel cells, electrolyzers, gas concentrators, gas compressors, sensors, supercapacitors, ultracapacitors, and industrial electrochemical process units.

For purposes of the present specification and claims, it is to be understood that certain relational terms used herein, such as “above,” “below,” “top,” “bottom,” “over,” “under,” “in front of,” or “behind,” when used to denote the relative positions of two or more components of an electrochemical device are used to denote such relative positions in a particular orientation and that, in a different orientation, the relationship of said components may be reversed or otherwise altered.

Additional objects, as well as features and advantages, of the present invention will be set forth in part in the description which follows, and in part will be obvious from the description or may be learned by practice of the invention. In the description, reference is made to the accompanying drawings which form a part thereof and in which is shown by way of illustration various embodiments for practicing the invention. The embodiments will be described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that structural changes may be made without departing from the scope of the invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is best defined by the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are hereby incorporated into and constitute apart of this specification, illustrate various embodiments of the invention and, together with the description, serve to explain the principles of the invention. In the drawings wherein like reference numerals represent like parts:

FIG. 1 is a schematic section view of a first embodiment of a solid polymer electrolyte composite membrane constructed according to the teachings of the present invention;

FIGS. 2( a) and 2(b) are fragmentary top and fragmentary perspective views, respectively, of the non-electrically-conductive support shown in FIG. 1;

FIG. 3 is a fragmentary top view of a non-electrically-conductive support that may be used as an alternative to the non-electrically-conductive support of FIGS. 2( a) and 2(b);

FIGS. 4( a) and 4(b) are schematic depictions of processes for preparing the non-electrically-conductive support of FIG. 1 by plasma dry etching; and

FIG. 5 is a schematic section view of a second embodiment of a solid polymer electrolyte composite membrane constructed according to the teachings of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring now to FIG. 1, there is shown a schematic section view of a first embodiment of a solid polymer electrolyte composite membrane constructed according to the teachings of the present invention, said solid polymer electrolyte composite membrane being represented generally by reference numeral 11.

Composite membrane 11 comprises a non-electrically-conductive support 13 and a solid polymer electrolyte 15, support 13 being impregnated with solid polymer electrolyte 15.

Referring now to FIGS. 2( a) and 2(b), support 13 can be seen to be a generally sheet-like, unitary structure, preferably of high mechanical strength, having a top surface 17 and a bottom surface 19. The thickness of support 13 may vary, depending upon the type of use to which membrane 11 is put and the types of pressures typically encountered by support 13 in such a use. For example, where membrane 11 is used in an electrolyzer, support 13 preferably has a thickness suitable for withstanding pressures of 2000-5000 psi. For most applications, support 13 has a thickness of about 5 μm to 50 μm, preferably about 7.5 μm to 15 μm.

As noted above, support 13 is preferably a rigid member; in addition, support 13 is preferably chemically resistant to acid and water hydrolysis at elevated temperatures. Examples of materials that may be used to make support 13 include, but are not limited to, perfluorinated polymers, polyvinylidene fluoride, poly(tetrafluoroethylene), polybenzimidazole, polyphenylenesulfide, polysulfone, polyethersulfone, polyesters, polyparaphenylene, polyquinoxaline, polyarylketone, polybenzazole, polyaramid, poly(etherether-ketone), liquid crystal polymers, polyimide and polyetherimide. A comparison of the mechanical strength of some of the above-listed polymers to conventional PEM materials is provided below in TABLE I.

TABLE I Material, condition Young's Modulus (Mpa) Nafion ® 112 PFSA membrane, dry 20° C. 300 Nafion ® 112 PFSA membrane, wet 80° C. 70 Poly(tetrafluoroethylene) (PTFE) 400 Polybenzimidazole (PBI) 5900 Polyphenylenesulfide (PPS) 3300 Polysulfone (PS) 2600 Poly(etherether-ketone) (PEEK) 2700 Polyimide (PI) 2900

As can be seen, PBI, PPS, PS, PEEK and PI are substantially stronger than PFSA and PTFE and, therefore, are preferred as materials for use in making support 13.

Kapton® polyimide (DuPont, Wilmington, Del.), which has a high strength, good hydrolysis stability and excellent thermal properties, is a particularly desirable material for use in making support 13. Ultra-thin membranes of Kapton® polyimide (8.5 μm and 17 μm) are commercially available and may be used to make support 13. VECTRA® liquid crystal polymers (Goodfellow, Cambridgeshire, UK) also have superb mechanical, chemical and thermal stability and may be used to make support 13.

Although the polymers discussed above as suitable for use in making support 13 are non-ionic, ionomers may alternatively be used. Examples of suitable cationic ionomers include carboxylated, sulfonated or phosphorylated derivatives of the polymers discussed above. Examples of suitable anionic ionomers include amino, imimo, ammonium, sulfonium and phosphonium derivatives of the polymers discussed above.

A plurality of pores 21, preferably cylindrical in shape, extend in a direct, i.e. straight-line, fashion from top surface 17 to bottom surface 19 of support 13. It should be stressed that the base shape of the pore can be chosen from any two-dimensional geometric shape distributed in either regular or irregular fashion. As will be discussed further below, pores 21 are made by plasma etching and preferably each have a diameter of about 0.1 μm to 200 μm, preferably about 0.5 μm to 10 μm, with pores 21 constituting about 5% to 95%, more preferably about 40% to 70%, of support 13.

The conductance of a membrane including such a porous support can be easily estimated as:

$\frac{1}{G} = {\frac{Ts}{\sigma*A*X} + \frac{\left( {T - {Ts}} \right)}{\sigma*A}}$ where G is the ionic conductance of the composite membrane, σ is the ionic conductivity of the solid polymer electrolyte, A is the geometric area of the composite membrane, X is the percentage of pores in the support, T is the thickness of the composite membrane and T_(s) is the thickness of the support.

As can be seen from the above equation, the conductance of the composite membrane is inversely proportional to the percentage of pores in the support. Thus, a support with 50% pores results in a composite membrane with conductance equivalent to a homogenous membrane twice as thick. To maximize the conductance of the supported membrane without a sacrifice in mechanical properties, a support with 50% pores may be fabricated.

In the present embodiment, pores 21 are arranged in a uniform hexangular pattern over the entirety of support 13, such pores 21 having, for example, a diameter of about 5 μm and a center-to-center spacing of about 7 μm. It is to be understood, however, that the present invention is not limited to the above-described pattern of pores and may encompass a variety of different patterns of pores. For example, as can be seen in FIG. 3, there is shown a fragmentary top view of a support 13′ having a plurality of pores 21′ that are arranged so that a lesser concentration of pores 21′ may be found in areas of higher membrane stress (e.g., at the membrane edge 23 or in local “hot spots” 25) and a greater concentration of pores 21′ may be found elsewhere.

Pores 21 (and 21′) are made by plasma etching. Referring to FIG. 4( a), there is schematically shown a first plasma etching process for forming pores 21 in support 13 (said process also being usable to form pores 21′ in support 13′), said process being represented generally by reference numeral 31.

As can be seen, process 31 begins in step 32 with the provision of a silicon wafer. (Other flat substrates including flat metal sheets may be used instead of a silicon wafer.) Next, as represented by step 33, a first thin layer of aluminum is sputtered onto the silicon wafer. Next, as represented by step 34, a layer of what will become support layer 13, preferably polyimide, is spin coated onto the aluminum layer. Next, as represented by step 35, a second thin layer of aluminum is sputtered onto the support layer precursor. Next, as represented by step 36, a layer of photoresist material is spin coated onto the second aluminum layer. Next, as represented by steps 37 and 38, the photoresist layer is imaged and then developed, thereby creating pores in the photoresist layer that expose the underlying second aluminum layer. Next, as represented by step 39, the exposed areas of the second aluminum layer are etched with acid, thereby creating pores in the second aluminum layer that expose the underlying support layer. Next, as represented by step 40, the photoresist layer is removed. Next, as represented by step 41, the exposed areas of the support layer are plasma etched. Finally, as represented by step 42, the aluminum layers are removed from the etched support.

It should be understood that, if desired, step 33 may be omitted entirely, with the support layer being deposited directly onto the silicon wafer (or other flat substrate used). Also, although spin coating is described above as being used to form the support and photoresist layers, direct coating processes may also be used to increase speed and lower cost. Examples of suitable coating techniques include gravure coating, immersion (dip) coating, metering rod (Meyer bar) coating, slot die coating, rotary screen and air knife coating.

One benefit of using plasma etching to create the pores in the support is that one can obtain pores in the micrometer scale, with an aspect ratio in the neighborhood of 8 to 10. At the same time, however, it should be noted that pores formed by plasma etching have tendency to diverge as they pass through a substrate (as opposed to being collimated). Consequently, such pores should not be spaced so closely to one another that they intersect.

Although process 31 is described herein as a batch process, the actual manufacturing process can readily be adapted to a continuous, fully-automated production line. Referring now to FIG. 4( b), there is schematically shown an example of a continuous manufacturing process, said process being represented generally by reference numeral 43.

Process 43 begins with the support precursor being provided in roll form, as represented by step 44. Next, as represented by step 45, a thin layer of aluminum is sputtered onto the support roll. Next, as represented by step 46, a layer of photoresist is coated onto the aluminum layer. Next, as represented by steps 47-1 and 47-2, the photoresist is image and developed, respectively, to expose portions of the underlying aluminum layer. Next, as represented by step 48, the aluminum layer is acid etched to expose portions of the underlying support layer. Next, as represented by step 49, the support layer is plasma etched. Finally, as represented by step 50, the aluminum layer is removed from the etched support.

Referring back to FIG. 1, solid polymer electrolyte 15 can be seen to fill pores 21 and to cover thinly top surface 17 and bottom surface 19 of support 13. Examples of suitable materials for use as solid polymer electrolyte 15 include (i) polymer compositions that contain metal salts; (ii) polymeric gels that contain electrolytes; and (iii) ion exchange resins. In general, if proton conductivity is required, a carboxylated, sulfonated or phosphorylated polymer is preferably used as solid polymer electrolyte 15. If hydroxyl ions are needed, a polymer containing amino, imimo, ammonium, sulfonium, and phosphonium groups is preferably used as solid polymer electrolyte 15. To enhance the ionic conductivity of membrane 11, inorganic ionically-conductive materials, such as metal oxide (e.g., TiO₂), silicon oxide, metal phosphates (e.g., zirconium phosphate) or heteropolyacids, may be impregnated into the solid polymer electrolyte 15.

A preferred material for use as solid polymer electrolyte 15 is a perfluorosulfonic acid (PFSA) membrane, such as is commercially available from DuPont (Wilmington, Del.) as NAFION® PFSA polymer. Of the aforementioned NAFION® PFSA polymers, particularly preferred are those having an equivalent weight of 200 to 2000, even more preferably those having an equivalent weight of 500 to 1200, the optimal equivalent weight depending on the use to which membrane 11 is applied.

Various techniques may be used to couple solid polymer electrolyte 15 to support 13. One such technique involves providing the solid polymer electrolyte in the form of a solution/dispersion (e.g., Nafion® 1100 in water or isopropanol) and then coating support 13 with said solution/dispersion. Examples of suitable coating techniques include gravure coating, immersion (dip) coating, metering rod (Meyer bar) coating, slot die coating, rotary screen and air knife coating. The optimal coating technique for any particular case will depend on factors, such as instrument complexity, thickness accuracy, operation efficiency, initial investment, and the like. After the solution/dispersion is coated onto the support, the coated support is heated at about 50° C. to 100° C., preferably 80° C., for about 5 minutes to evaporate the solvent. If desired, one or more additional coatings may thereafter be applied. Said one or more additional coatings either may be of the same solution/dispersion previously applied in order to build up the thickness of the solid polymer electrolyte or may be different from the initial solution/dispersion in order to obtain a composite membrane with a multilayer electrolyte structure having desired properties. (An example of a composite membrane possessing such a multilayer structure is shown in FIG. 5, said composite membrane 51 comprising a support 13, a first solid polymer electrolyte 53 and a second polymer electrolyte 55.) After coating and drying each applied layer or after coating and drying all applied layers, the layer or layers are preferably cured by heating at a temperature greater than the glass transition temperature of the ionomer (e.g., 100° C. to 400° C., preferably 160° C. for 15 minutes). Such curing, which serves to sinter or anneal the ionomer, further enhances the mechanical properties of the membrane.

If desired, mask coating technology may be used to create a composite membrane wherein the solid polymer electrolyte is confined to certain patches or regions.

As an alternative to coating, the solid polymer electrolyte may be applied by spraying the polymer electrolyte solution/dispersion onto support 13. Conventional spraying techniques may be used for this purpose. Such spraying is preferably performed at 80° C. and does not require a subsequent solvent evaporation step. Micro-spraying may be used to create solid polymer electrolyte patches on the support, such patches, if desired, being far smaller than those capable of being produced by masked coating techniques.

Still another technique for incorporating the solid polymer electrolyte 15 into support 13 involves a membrane extrusion technique. Such a technique comprises providing the solid polymer electrolyte in the form of a thin membrane, stacking the thin ionomer membrane on the top and/or bottom surfaces of support 13, and then pressing the stack together at an elevated pressure, preferably above the melting point or glass transition temperature of the ionomer.

The total thickness of membrane 11 is preferably about 5 to 300 μm, more preferably 10 to 75 μm, with the thickness of the membrane being governed by application requirements. Generally, a water electrolyzer requires a thicker membrane due to its high differential pressure while an ultra-thin membrane is suitable for super capacitors since no dynamic pressure is involved for their operation.

Membrane and electrode assemblies (MEAs) comprising the composite membrane of the present invention can be fabricated by pressing a precast Pt-supported on carbon/ionomer ink onto each side of the composite membrane. The foregoing method is typically referred to as the decal transfer method.

One of the advantages of the composite membrane of the present invention is that catalyst ink can be directly coated or sprayed onto the top and bottom surfaces of the membrane. This direct coating of the catalyst ink is not practical for conventional ionomer membranes since such membranes change dimension when contacted with the ink. Since the composite membrane of the present invention has excellent dimensional stability when contacted with swelling agents, such as water or alcohols, the catalyst ink can be directly applied to the membrane.

Another approach that may be used to fabricate an MEA using the composite membrane of the present invention involves a technique called “catalyst on diffusion media.” The diffusion medium of this technique is a porous electrically-conductive material, which is typically in the form of a thin sheet. Optionally, a microporous layer prepared from carbon black and a polymer binder may be applied to the diffusion medium. The catalyst is then sprayed onto the diffusion medium to form a diffusion electrode. A catalyst-loaded diffusion electrode is then pressed onto each side of the composite membrane to form a full MEA.

Such an MEA may be used, for example, by being sandwiched between two pieces of teflonated porous carbon paper and assembled into a functional PEM fuel cell as described in U.S. Pat. No. 4,215,183, inventor MacLoed, which issued Jul. 29, 1980, the disclosure of which is incorporated herein by reference, or by being sandwiched between porous titanium metal meshes or sinters and assembled into an electrolysis cell as described in U.S. Pat. No. 6,500,319, inventors LaConti et al., which issued Dec. 31, 2002, the disclosure of which is incorporated herein by reference.

The embodiments of the present invention recited herein are intended to be merely exemplary and those skilled in the art will be able to make numerous variations and modifications to it without departing from the spirit of the present invention. All such variations and modifications are intended to be within the scope of the present invention as defined by the claims appended hereto. 

1. A method of preparing a composite membrane, said method comprising the steps of: (a) providing a non-electrically-conductive support, said non-electrically-conductive support being made of a polymeric material; (b) plasma etching a plurality of pores through said non-electrically-conductive support, wherein at least some of said pores are located in a peripheral portion of said non-electrically-conductive support, wherein at least some of said pores are located in a non-peripheral portion of said non-electrically-conductive support, and wherein a greater concentration of said pores are located in said non-peripheral portion than in said peripheral portion; and (c) at least partially filling at least some of said pores with a first solid polymer electrolyte.
 2. The method as claimed in claim 1 wherein said non-electrically-conductive support is a rigid member.
 3. The method as claimed in claim 1 wherein said plurality of pores are arranged in a non-random pattern.
 4. The method as claimed in claim 3 wherein said plurality of pores are arranged in a hexangular pattern.
 5. A method of preparing a composite membrane, said method comprising the steps of: (a) providing a non-electrically-conductive support, said non-electrically-conductive support being made of a polymeric material; (b) plasma etching a plurality of pores through said non-electrically-conductive support, wherein said pores are unevenly distributed throughout said non-electrically-conductive support, with some of said pores being positioned in a peripheral portion of said non-electrically-conductive support and some of said pores being positioned in a non peripheral portion of said non-electrically conductive support, said pores being positioned in a greater concentration in said non-peripheral portion than in said peripheral portion; and (c) at least partially filling at least some of said pores with a first solid polymer electrolyte.
 6. The method as claimed in claim 1 wherein said pores are generally cylindrical in shape.
 7. The method as claimed in claim 1 wherein said pores have a diameter of about 0.1 to 200 microns.
 8. The method as claimed in claim 7 wherein said pores have a diameter of about 0.5 to 10 microns.
 9. The method as claimed in claim 1 wherein said pores constitute about 5% to 95% of said non-electrically-conductive support.
 10. The method as claimed in claim 9 wherein said pores constitute about 40% to 70% of said non-electrically-conductive support.
 11. The method as claimed in claim 1 wherein said non-electrically-conductive support is made of a polymer selected from the group consisting of perfluorinated polymers, polyvinylidene fluoride, poly(tetrafluoroethylene), polybenzimidazole, polyphenylenesulfide, polysulfone, polyethersulfone, polyesters, polyparaphenylene, polyquinoxaline, polyarylketone, polybenzazole, polyaramid, poly(etherether-ketone), liquid crystal polymers, polyetherimide and polyimide.
 12. The method as claimed in claim 11 wherein said non-electrically-conductive support is made of polyimide.
 13. The method as claimed in claim 1 wherein said non-electrically-conductive support has a thickness of about 5 to 50 microns.
 14. The method as claimed in claim 13 wherein said non-electrically-conductive support has a thickness of about 7.5 to 15 microns.
 15. The method as claimed in claim 1 wherein said first solid polymer electrolyte is selected from the group consisting of polymer compositions that contain metal salts, polymeric gels that contain electrolyte and ion exchange resins.
 16. The method as claimed in claim 15 wherein said first solid polymer electrolyte is an ion exchange resin.
 17. The method as claimed in claim 15 wherein said first solid polymer electrolyte is perfluorosulfonic acid (PFSA) polymer.
 18. The method as claimed in claim 17 wherein said PFSA polymer has an equivalent weight of about 200 to
 2000. 19. The method as claimed in claim 18 wherein said PFSA polymer has an equivalent weight of about 500 to
 1200. 20. The method as claimed in claim 1 wherein said filling step comprises completely filling at least some of said pores with said first solid polymer electrolyte, said method further comprising positioning a second solid polymer electrolyte at least one of over the top and under the bottom of said first solid polymer electrolyte.
 21. A method of preparing a composite membrane, said method comprising the steps of: (a) providing a non-electrically-conductive support, said non-electrically-conductive support being made of a polymeric material; (b) plasma etching a plurality of pores through said non-electrically-conductive support wherein at least some of said pores are located in a peripheral portion of said non-electrically-conductive support, wherein said pores include a first group of pores and a second group of pores, said second group of pores being located around the periphery of said first group of pores, said first group of pores being arranged in a density greater than that of said second group of pores; (c) at least partially filling at least some of said pores with a first solid polymer electrolyte, wherein said filling step comprises completely filling at least some of said pores with said first solid polymer electrolyte; and (d) positioning a second solid polymer electrolyte at least one of over the top and under the bottom of said first solid polymer electrolyte, wherein said second solid polymer electrolyte differs from said first solid polymer electrolyte.
 22. The method as claimed in claim 20 wherein said second solid polymer electrolyte is positioned over the top and under the bottom of said first solid polymer electrolyte.
 23. The method as claimed in claim 1 wherein said composite membrane has a thickness of about 5 to 300 microns.
 24. The method as claimed in claim 23 wherein said composite membrane has a thickness of about 10 to 75 microns.
 25. The method as claimed in claim 1 wherein said non-electrically-conductive support is non-electrolytically-conductive.
 26. The method as claimed in claim 1 wherein said non-electrically-conductive support is electrolytically-conductive.
 27. The method as claimed in claim 26 wherein said non-electrically-conductive support comprises an anionic polymer electrolyte.
 28. The method as claimed in claim 26 wherein said non-electrically-conductive support comprises a cationic polymer electrolyte.
 29. The method as claimed in claim 1 wherein said first solid polymer electrolyte is impregnated with an inorganic ionic conductive materials.
 30. The method as claimed in claim 1 wherein said non-electrically-conductive support has a Young's Modulus of at least approximately 2500 Mpa.
 31. The method as claimed in claim 1 wherein said pore filling step comprises providing said first solid polymer electrolyte in a solution, then coating said solution into said pores of said non-electrically-conductive support, evaporating any solvent in said solution and then curing said first solid polymer electrolyte.
 32. The method as claimed in claim 1 wherein said pore filling step comprises providing said first solid polymer electrolyte in a solution and then spraying said solution into said pores of said non-electrically-conductive support.
 33. The method as claimed in claim 1 wherein said pore filling step comprises providing said first solid polymer electrolyte as a membrane and then pressing said membrane against said non-electrically-conductive support at a temperature above the glass transition temperature of the first solid polymer electrolyte.
 34. The method as claimed in claim 1 wherein said filling step comprises completely filling at least some of said pores with said first solid polymer electrolyte, said method further comprising depositing a layer of a second solid polymer electrolyte at least one of over said top surface and under said bottom surface of said first solid polymer electrolyte.
 35. The method as claimed in claim 34 wherein said first and second solid polymer electrolytes are different.
 36. A method of preparing a composite membrane, said method comprising the steps of: (a) providing a non-electrically-conductive support, said non-electrically-conductive support being made of a polymeric material; (b) plasma etching a plurality of pores through said non-electrically-conductive support, wherein at least some of said pores are located in a peripheral portion of said non-electrically-conductive support and at least some of said pores are located in a non-peripheral portion of said non-electrically-conductive support, and wherein said pores constitute about 70% to 95% of said non-electrically-conductive support; and (c) at least partially filling at least some of said pores with a first solid polymer electrolyte.
 37. The method as claimed in claim 36 wherein said pores constitute about 95% of said non-electrically-conductive support.
 38. The method as claimed in claim 36 wherein said pores constitute about 70% of said non-electrically-conductive support. 